Challenges of UHF RFID System Testing

RFID is a low-power, short-range wireless communication technology, and its full name is "Radio Frequency Identification". The composition of an RFID system generally includes at least two parts: an electronic tag (Tag in English) and a reader/writer (Reader/Writer or Interrogator in English). Electronic tags generally store electronic data in a specified format. In practical applications, electronic tags are attached to the surface of the object to be identified. The reader/writer can read and identify the electronic data stored in the electronic tag without contact, thereby achieving the purpose of automatically identifying the object. Further management functions such as the collection, processing and remote transmission of object identification information are realized through computers and computer networks.

RFID technology presents several unusual engineering test challenges, such as transient signals, bandwidth-inefficient modulation techniques, and backscatter data. Traditional swept-tuned spectrum analyzers, vector signal analyzers, and oscilloscopes have been used in the development of wireless data links. However, these tools have some shortcomings when used for RFID testing. Swept-tuned spectrum analyzers have difficulty accurately capturing and characterizing transient RF signals. Vector signal analyzers do not actually support spectrally inefficient RFID modulation techniques and special decoding requirements. Fast oscilloscopes have a small measurement dynamic range and do not have modulation and decoding capabilities. Real-time spectrum analyzers (RTSAs) overcome the limitations of these traditional test tools, have optimization for transient signals, and can reliably trigger specific spectrum events in complex real-world spectrum environments through Tektronix's patented frequency mask trigger.

The test of RFID signals covers the frequency domain, time domain and modulation domain analysis of radio frequency signals. For chip and equipment R&D and manufacturing companies, it is necessary to test whether the products comply with radio regulations and industry standards. For system integrators, it is necessary to analyze the interoperability of RFID systems in actual environments (for example, analyzing the interference between readers, tags and the surrounding environment, optimizing the placement of readers and writers, and reasonably adding shielding materials, etc.). These requirements require the test instrument to have the following characteristics:

Standards compliance testing

The signal is intermittent and has low power, and sometimes even a frequency-hopping signal. The test instrument needs to be able to capture transient change signals, store the signal for a long enough time seamlessly and replay it for analysis, and have a fast test speed. The signal uses a variety of different modulation methods, which requires the test instrument to have digital modulation signal analysis capabilities. It is best to support various RFID standards, not only to be able to demodulate, but also to be able to decode a variety of codes to read data.

The signal encoding specifies many time parameters. The reader and tag emission and response time parameters also need to be tested, requiring the test instrument to have signal time domain analysis function, multi-domain correlation analysis capability and good time resolution. Many RFID systems use frequency hopping system, and the test instrument must have the ability to analyze frequency hopping signals. Limitations of using traditional instruments: To use traditional instruments to test the standard compliance of RFID products, we need multiple devices, but many projects still cannot be tested well. For example, using a swept spectrum analyzer, it is difficult to capture the intermittent signal of RFID, unable to analyze the modulation characteristics of the signal, and unable to analyze the time domain characteristics of a complete communication sequence. Using a vector signal analyzer, limited by limited triggering methods, makes it extremely cumbersome to find RFID data packets, especially for RFID systems with frequent carrier transmission; and limited time correlation multi-domain, it is difficult to observe the characteristics of modulation characteristics and frequency characteristics changing with time. Using an oscilloscope, we have to face difficulties such as poor dynamic range, extremely limited frequency domain function, and inability to perform modulation analysis.

Interoperability testing of RFID systems

When the RFID system is installed in the actual environment, it is often affected by various factors, such as the influence of the surrounding electromagnetic environment, the mutual interference between multiple readers and multiple tags, and the interference between high-frequency and low-frequency RFID systems, which can cause the RFID system to fail to work or reduce its performance. In order to solve the above problems, testing is required.

The device has the following capabilities:
1 Real-time observation of the status of readers and tags in all RFID channels.
2 Analyze the detailed communication between tags and readers, such as carrier sensing and transmission control in all channels.
3 With high sensitivity, it can grasp the reader signals in other areas and the electromagnetic interference in the surrounding environment, and has the ability to capture steady-state interference signals and transient interference signals.
4 Capture RFID signals in the actual environment and observe the modulation quality.
5 Limitations of traditional instruments:

Analyzing RFID signals and interference signals in real environments is a very difficult task for traditional equipment. When using a swept spectrum analyzer, it is impossible to observe the conditions of each channel in real time, detect transient electromagnetic interference, and analyze the modulation quality. Using a vector signal analyzer does not have a true real-time feature, and it is also impossible to observe and detect transient interference phenomena in real time. [page]

Tektronix's latest RSA6100 series provides unique DPX digital fluorescence technology, which uses color temperature to indicate the frequency of signal occurrence. Different signals of the same frequency are displayed separately in "real time" according to the probability of occurrence. The occupancy rate of each channel can also be distinguished by color, and 100% of transient changes greater than 24us can be found. This allows users to observe the situation in the communication system in real time.

The application of real-time spectrum analyzer in RFID testing The characteristics of real-time spectrum analyzer are very suitable for the testing of RFID equipment, providing a new and complete RFID testing solution. Seamless capture ensures the capture of the fastest changing pulse signal, powerful analysis capability ensures that any part of the signal is fully analyzed, and multi-domain analysis capability ensures complete analysis of the time, frequency and modulation characteristics of the signal. [page]

Capturing RFID signals with a real-time spectrum analyzer
To analyze RFID signals, you must first be able to capture them. Traditional instruments, such as vector signal analyzers, can only blindly shoot signals and then search for useful signal segments in the captured data. Real-time spectrum analyzers, on the other hand, can use frequency mask triggering to purposefully and quickly capture signal segments of interest.

Compliance testing for government radio regulations

Government regulations require that transmitted signals be controlled in power, frequency, and bandwidth. These regulations prevent harmful interference and ensure that each transmitter is a good neighbor to other users of the band. RTSAs with RFID software can easily measure the spectrum parameters required by government regulations. Power measurements on pulsed signals can be challenging for many spectrum analyzers. The RTSA's transient signal optimization feature makes it easy to measure the power in pulsed RFID packet transmissions. FFT analysis provides a complete spectral frame for any given time period during the packet transmission, without the need to synchronize the tuning scan with the packet burst as with older swept spectrum analyzers. In addition, traditional spectrum analyzers require correction factors to compensate for the continuous log video amplifier (SLVA) peak detection circuit, while RTSAs use true RMS detection to accurately read power for most regulatory measurements.

Another important spectrum emission measurement item is the carrier frequency of the signal. This measurement can be expressed in two ways: the actual absolute carrier frequency or the carrier frequency error of a certain assigned channel frequency. When demodulating a signal, the RTSA will display the carrier frequency error. In spectrum analysis mode, you can select the measurement button and then use the carrier frequency soft key to display the absolute carrier frequency. A significant advantage of demodulated carrier frequency measurement
is that it does not require the signal to be located in the center of the span. This is particularly suitable for frequency hopping signals. Similarly, occupied bandwidth (OBW) or radiated bandwidth (EBW) can also be obtained in two ways. In demodulation mode, the RTSA displays OBW and EBW along with the carrier frequency and transmission power level. In real-time spectrum analyzer mode, bandwidth measurements are also provided under the measurement key. In RTSA, each RFID test has a corresponding measurement button, which can easily view various parameters. Figure 5-16 shows the measurement of carrier frequency. Similarly, parameters such as channel power, occupied bandwidth, radiated bandwidth, interrogator spurious emissions (INTERROGATOR TRANSMIT SPURIOUS EMISSIONS), ACPR, etc. can also be measured intuitively by pressing the corresponding button.

The National Industry and Information Technology Bureau's "Notice on the Release of Trial Provisions on the Application of Radio Frequency Identification (RFID) Technology in the 800/900MHz Frequency Band" stipulates:

1. The specific use frequency of RFID technology in the 800/900MHz frequency band is 840-845MHz and 920-925MHz.
2. RF indicators of radio transmitter equipment for RFID technology in this frequency band:
1. Carrier frequency tolerance: 20×10-6;
2. Channel bandwidth and channel occupied bandwidth (99% energy): 250kHz;
3. Channel center frequency:
fc (MHz) = 840.125 + N × 0.25 and
fc (MHz) = 920.125 + M × 0.25 (N, M are integers, ranging from 0 to 19);
4. Adjacent channel power leakage ratio: 40dB (first adjacent channel), 60dB (second adjacent channel);
5. Transmitting power:

6. The working mode is frequency hopping spread spectrum, and the maximum dwell time of each frequency hopping channel is 2 seconds;

7. Spurious emission limits (outside the range of ±1MHz between the middle carrier frequencies of the two frequency bands):

Government regulations require controls on the power, frequency, and bandwidth of transmitted signals. These regulations prevent harmful interference and ensure that each transmitter is a friendly neighbor to other users of the frequency band. Making such measurements is challenging for many spectrum analyzers, especially swept spectrum analyzers that are typically used for energy measurements on pulsed signals. RTSAs can analyze the energy characteristics of a complete packet transmission and can directly measure the carrier frequency of frequency hopping signals without centering the signal on a span. At the touch of a button, the analyzer can identify the modulation of a transient RFID signal and make regulatory measurements of power, frequency, and bandwidth, making the pre-compliance testing process very flexible and convenient. Pre-compliance testing helps ensure that products pass compliance testing the first time without the need for redesign and retesting.